First make sure your RPM readings are correct. With diesels, unless you have a mechanical takeoff, there can be lots of errors. Most tachs operate by measuring the frequency of the AC generated in the alternator. This requires calibration by an external reference. Best option is an optical tach. These operate by putting a reflective patch on the flywheel, shining a light on it and counting the returning light pulses. I got one off EBay for about $40.

If your RPM measurements are correct, fire the mechanic. You most likely have a problem with the govenor.

The 3JH2 Yanmar tachometer uses a magentic pickup sender that converts the rotary motion into an electrical signal by counting the number of teeth (127) of the ring gear that is connected to the flywheel. PP 9-42 Yanmar Service Manual.